Electronic devices are widely used in people's life and work environment now. Those electronic devices mostly include a lot of different electronic elements connecting together to achieve various purposes. The electronic elements usually generate heat during operation and result in a lower efficiency or even operation interruption. Hence heat sinks are commonly adopted on the heated electronic elements or nearby to quickly dissipate the heat generated during operation of the electronic elements to lower operation temperature and ensure regular operation thereof. To meet such a purpose, cooling fans are widely adopted. And achieving a higher cooling efficiency of the cooling fans is an important issue. Conventional focus on this issue usually aims to alter blade and frame structures, or increase air fan rotation speed to enhance intake airflow amount and exit airflow amount to boost cooling efficiency. However, increasing airflow amount and air fan rotation speed generate greater impact and friction between the air and blades during air fan operation, and produce greater operational noise caused by wind shearing. As a result, the cooling effect of the air fan suffers.
To remedy the aforesaid problems, the techniques of adopting a flow directing structure on the air fan blades to channel airflow motion have been proposed in prior art. For instance, R.O.C. patent No. 590272 discloses an improved cooling fan which has a plurality of blades arranged on the periphery of the axis of an air fan in an annular array fashion. Each of the blades has a bottom surface formed with a plurality of jutting ribs arched towards the axis. When the air fan rotates, the blades generate cooling airflow blowing downwards. The jutting ribs direct the cooling airflow below the axis to prevent generation of turbulence and noises, and also to increase cooling efficiency.
Another R.O.C. patent No. 595661 discloses improved blades for an extractor fan. It has a hub in the center and a plurality of blades around the hub. The blades are spaced from each other with a desired gap. Each blade has an inner surface on which jutting directing ridges are formed in the rotation direction. Between the ridges a flow directing portion is formed. The blade further has an inward concave portion at one side corresponding to the flow directing direction. When the blades rotate, air is converged along the flow directing portion and channeled to the inward concave portion and discharged through the gap between the blades and the inward concave portion. Such a structure can steady airflow and reduce the impact noise of the air and the blades. Moreover, the inward concave portion can quickly direct discharge of the airflow, thus improve air discharge efficiency and lower the noise.
China patent No. CN1590778 discloses an axial flow fan which has a pressure surface on the blades. The axial flow fan has ribs extended from the front side thereof to the rear side of the blades in parallel with the rotation track of the blades to direct airflow during operation.
All the conventional techniques mentioned above have a common feature: namely the airflow directing structure of the air fan is located at the bottom surface of the blades (i.e. airflow exit surface) to direct and form airflow. In practice, airflow first hits the top surface of one blade (i.e. air intake surface), then hits the bottom surface of a neighboring blade due to air fan rotation. As the aforesaid conventional techniques have the airflow directing structure formed merely on the bottom surface of each blade, the blade receives an uneven force. Not only operational noise problem cannot be improved, cooling efficiency also is lower due to poorer airflow directing effect.